This invention relates generally to pigments, and more specifically relates to a process for producing high brightness pigments by beneficiation of naturally occurring calcium carbonate.
Calcium carbonate pigments find application in a wide variety of industrial and other environments. Such pigments, for example, are widely utilized as fillers in the manufacture of rubber, paper, and of various plastics, and as extenders in paint formulations. Such pigments, furthermore, either alone or in combination with other pigments, are widely utilized for paper coating applications. In many of the aforementioned applications, particularly e.g. where the pigments are used for coating, it is desirable that the calcium carbonate be as bright as possible.
In addition it is desirable in many applications, to utilize a calcium carbonate of high purity, for reasons other than brightness alone. For example, in the preparation of certain insulating compositions, calcium carbonate is used as an extender; and if metallic impurities such as pyrites are present, the electronic properties of the product can be adversely affected.
High purity, high brightness calcium carbonate pigments have long been produced by chemical processes, pursuant to which the said carbonates are prepared as precipitates. The processes, however, are comparatively complex, and are not well-suited to large scale or low cost operations. Interest for such reason has long centered upon possible use of naturally occurring calcium carbonate, particularly in that the natural calcitic minerals are extremely abundant in virtually all parts of the world, -- and therefore represent a ready source of inexpensive raw material. In practice, however, it is found that very many of the calcitic deposits are so highly contaminated with discolorants, that when comminuted in their natural state they are simply inacceptable as pigments. Thus, in typical instances deposits which are primarily of calcite, may be contaminated with pyrites and with mica; both of which in varying degrees contribute to the discoloration of the otherwise relatively colorless calcium carbonate.
Various proposals have from time to time been advanced for processes directed at improving the brightness of the aforementioned naturally occurring calcium carbonates. Many of these prior proposals involve the use of flotation methods to remove impurities from calcitic ores. In the bulk of such prior art, interest has centered upon the removal of siliceous impurities, so as to improve the remaining compositions for use in cement manufacture.
In some instances too, flotation is recommended for improving the brightness characteristics of naturally occurring calcium carbonate. Thus in U.S. Pat. No. 3,512,722 a wetground natural calcium carbonate is subjected to a flotation step, after which the treated product is classified and further processed.
In some other instances, for example, in U.S. Pat. No. 2,287,440, it has been contemplated that magnetic separation techniques might be used to beneficiate naturally occurring calcitic ores, so as to improve the brightness thereof. In the case of the cited patent, the ore is ground and crushed, and then heated as a dry powder to increase the magnetic susceptibility. The dry powder is then subjected to magnetic separation, after which the purified product is wet-milled to produce the improved pigment.
By and large the techniques of the prior art as discussed, have not been particularly effective in producing high brightness calcium carbonate pigments from natural sources. Indeed in most instances where pigments are derived on a commercial scale from such natural sources, an ore is utilized which to begin with is of very high purity and relatively free from discoloring contaminants. Reference may be had in this connection, for example, to U.S. Pat. Nos. 3,661,610 and 3,674,529, which cite the use of calcium carbonate pigments derived from a raw, high purity natural chalk whiting. The natural whiting is subjected to a two-stage grinding process, it not being necessary in these instances to subject the product to purification steps.
In accordance with the foregoing, it may be regarded as an object of the present invention, to provide a process which enables the production of high quality bright calcium carbonate pigments, from naturally occurring calcitic ores containing relatively high levels of discoloring contaminants.
It is a further object of the present invention, to provide a process which enables notable brightness augmentation, even for natural calcitic deposits which are initially relatively free of discoloration.